This invention is concerned with a process for producing polymethyl methacrylate.
Polymethyl methacrylate (hereinafter called "PMMA"), a resin with the best transparency among plastics, has long been used as windshields and in other similar applications. Since it is a material of a long history, various production methods depending on polymerization have been proposed. For the manufacture of the resin as an optical link material, however, none of them have proved satisfactory.
Among the proposed methods, first comes batch suspension polymerization that consists of polymerizing monomer particles batchwise in an aqueous dispersion and suspension containing a polymerization initiator. The process is still in wide use, but the batch operation usually requires a polymerization initiator (catalyst) which is eventually left unremoved in the product polymer, together with a suspending agent and the like, and the molecular weight distribution broadens. Also, the process necessarily involves such open stages as dehydration and drying which allow intrusion of impurities to mar the transparency of the product.
The second method comprises first preparing a syrupy intermediate polymer by batch bulk polymerization, placing the syrup in between two sheets of glass and effecting polymerization, and, following the conclusion of the polymerization, peeling off the glass sheets to obtain a sheet of PMMA. The process again needs a polymerization initiator and, because the polymerization is carried out while the resulting polymer is allowed to cool naturally between the glass sheets, the polymerization temperature becomes nonuniform and the polymerization degree distribution spreads objectionably. The result is poor transparency of the product.
The third is continuous bulk polymerization that can theoretically afford the most transparent product. According to the process disclosed, for example, by Japanese Patent Application Publication No. 32665/1977, a polymerization initiator is used and a syrupy intermediate polymer is formed in a prepolymerization vessel, and then in a second-stage polymerization vessel the intermediate is further polymerized to a final polymerization ratio .phi..gtoreq.0.5. The two polymerization vessels use different polymerization temperatures, and this results in molecular weight distribution and compositional distribution as will be described later. Moreover, at .phi..gtoreq.0.5, a gel effect accelerates the polymerization, and the rapid progress of polymerization causes partial temperature distribution and hence widened molecular weight distribution. Consequently, the product will not attain adequate transparency as an optical link material.
Generally, the molecular weight of PMMA is a function of the polymerization temperature and the amounts of the polymerization initiator, chain transfer agent such as ethylbenzene, and molecular weight modifier such as mercaptan contained in the feedstock. Together with proper amounts of these additives, the monomeric material is passed through a high-precision filter to a particle-free state. The mixture is then fed to a reactor and polymerization is carried out with stirring. In the case of batch or continuous [plug (piston) flow type] bulk polymerization, the reaction is effected at relatively low temperatures because the initial polymerization velocity is so high that the heat of polymerization is difficult to dissipate. The syrupy intermediate polymerization product thus obtained is transferred to a polymerization reactor, where the rest of polymerization reaction is performed.
FIG. 1 i a graphic representation of changes in the rates of reaction with time at different temperatures T.sub.1, T.sub.2, and T.sub.3. The experiments showed that, with the same polymerization reactor and second-stage reactor, the polymerization reaction proceeded with sequential shifts of the temperature from the curve T.sub.1 to T.sub.2 and thence to T.sub.3. Toward the end of reaction a gel effect accelerated the reaction, raising the rate of reaction to excess. Thus, the molecular weight distribution in the product PMMA spreads out to impair the transparency. The initiator and other additives partly left unremoved are also responsible for the low transparency of the product of ordinary bulk polymerization.
As described above, the conventional PMMA resins have not been satisfactory for high-precision applications such as optical fibers for optical links.